SAUCE_USERNAME and SAUCE_ACCESSKEY are the username and the accesskey for saucelabs. They are usually set as environment variables (specially in continuous integration tools like travis )

The tunnelIdentifier is a unique identifier for the tunnel. It is optional and is automatically generated when not specified. Note that the tunnel identifier may have to be passed in with the browsers object as a desired capability to enable traffic to use the tunnel. More details here

The tunneled attribute is a boolean value to indicate if the tunnel is to be created or not. This value can be set to false to mock a tunnel creation if the site tested is publicly accessible. This value is optional and defaults to true.

The extraFlags attribute is an array of options flags (see here). Example: ['--debug', '--direct-domains', 'www.google.com']. It is optional.